There's an alternate form of the third person plural perfect active indicative. Instead of, say, habuerunt, a poet might write habuere, to make the word fit with the meter, but that looks like the infinitive! How can I tell them apart?
Three considerations are helpful to distinguish between the infinitive and the alternative 3rd person plural perfect ending:
- -ēre is added to the perfect (3rd) stem, while the present infinitive is formed on the present (1st) stem. Although the perfect infinitive is also formed on the 3rd stem, its form (isse) is easily distinguishable.
- -ēre has a long ē vowel. Besides the second conjugation (e.g. vidēre), all other (common) verbs are formed with an -a (1st conjugation, e.g. laudare), a short -e (3rd conjugation -o and -io, e.g. mittere and capere), or an -i (4th conjugation, e.g. audire).
- Context! I cannot think of (though there probably are examples of...) a grammatical Latin sentence where an infinitive or a 3rd person perfect verb could be swapped without doing violence to the syntax.
There are cases where (1) and (2) will fail, as in the case of vidēre which happens to have the same vowel length and the same 1st and 3rd stem. In this case, context is key: Regressi eum vidēre only makes sense where vidēre is perfect. Te vidēre volo only makes sense as an infinitive.
Regarding your footnote, yes, this contraction only normally exists in the active indicative. The perfect subjunctive (e.g. viderint) has no contraction and the perfect passive (both indicative and subjunctive) are compounds formed with the fourth principle part (visi sunt, sint).
It is very rarely the case that the alternative form of the perfect in third person plural really looks like present infinitive. If you know how to conjugate your verbs, there is rarely a problem.
In the second conjugation this happens with the following words (I have only listed the relevant forms):
- moveō, mōvī (with possible prefixes)
- sedeō, sēdī (one-syllable prefixes weaken the present stem but not the perfect stem, so ambiguity is lost: assideō, assēdi but circumsedeō, circumsēdī)
- attondeō, attondī (reduplication in the absence of prefixes: tondeō, totondiī)
- caveō, cāvī
- faveō, fāvī
- foveō, fōvī
- videō, vīdī (with possible prefixes)
- respondeō, respondī (reduplication without prefixes: spondeō, spopondī)
- strīdeō, strīdī
The only ones of these that have the same vowel lengths and are thus metrically indistinguishable in verse, are attondere, respondere and stridere.
In the third conjugation the infinitive always has a short -e- before the ending -re, so vowel length can always tell them apart. In the absence of macrons there are more look-alikes in third conjugation than in the second one, so I will not list them here. (I might do so later.)
If I missed some verbs, let me know. I'm travelling now and I don't have access to my printed dictionary that I would love to use here.
There are two main ways. The first is context; if there's no other main verb, by process of elimination, the word in question has to be. This is typically how I figure it out; I'll read the sentence taking it as an infinitive, realize there's no main verb, and go back to look at all of the candidates.
The second is that this specific kind is an abbreviation, so it uses the same stem as the full form -- namely, the perfect. Notice the difference between habeo, habere's infinitive and this form:
One rule of thumb to tell if it's the perfect stem instead of present is that present stems rarely end in -u, -v, or -x, while perfect stems often do. However, there are many exceptions to this rule -- for example, the present stems of abluere, induere, salvere, exere, and mixere, and the perfect stems of such as bibi, feci, egi, arsi, cecini.1
1: Source: C.M.Weiner